Onychomycosis—nail fungal infection—affects 30-60 million patients each year in the United States. It is the most common disease of the nails and constitutes about a half of all nail abnormalities. This condition may affect toenails or fingernails, but toenail infections are particularly common. The prevalence of onychomycosis is about 6-8% of the United States adult population. Common signs of onychomycosis include a thickened, yellow, or cloudy appearance of the nails. The nails can become rough and crumbly, and can separate from the nail bed. Patients with onychomycosis may experience significant psychosocial problems due to the appearance of the nail.
The causative pathogens of onychomycosis include dermatophytes, Candida, and nondermatophytic molds. Dermatophytes are the fungi most commonly responsible for onychomycosis in the temperate western countries, while Candida and nondermatophytic molds are more frequently involved in the tropics and subtropics with a hot and humid climate. Trichophyton rubrum is a common dermatophyte involved in onychomycosis. Other dermatophytes that may be involved are T. mentagrophytes, Epidermophyton floccosum, T. violaceum, Microsporum gypseum, T. tonsurans, T. soudanense and the cattle ringworm fungus T. verrucosum. Other causative pathogens include Candida and nondermatophytic molds, in particular members of the mold generation Scytalidium (now called Neoscytalidium), Scopulariopsis, and Aspergillus. Candida spp. are known to cause fingernail onychomycosis in people whose hands are often submerged in water. Scytalidium mainly affects people in the tropics, though it persists if they later move to areas of temperate climate. Control of these pathogens can be used to treat onychomycosis.